Curriculum Vitaes
Profile Information
- Affiliation
- Associate professor, Institute of Space and Astronautical Science, Japan Aerospace Exploration AgencyAssociate professor, Space and Astronautical Science, Graduate Institute for Advanced Studies, The Graduate University for Advanced Studies
- Degree
- Ph. D.(Mar, 1998, Kyoto University)master's degree(Mar, 1995, Kyoto University)
- Researcher number
- 80342624
- ORCID ID
https://orcid.org/0000-0002-9099-5755- J-GLOBAL ID
- 202001011170717781
- researchmap Member ID
- R000011919
Research Areas
2Research History
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Apr, 2026 - Present
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Apr, 2007 - Mar, 2026
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Oct, 2003 - Mar, 2007
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Jul, 2001 - Sep, 2003
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Apr, 2001 - Jun, 2001
Education
3-
Apr, 1995 - Mar, 1998
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Apr, 1993 - Apr, 1995
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Apr, 1989 - Mar, 1993
Papers
279-
Journal of Astronomical Telescopes, Instruments, and Systems, 12(02), May 22, 2026
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The Astrophysical Journal, 1002(2) 222-222, May 8, 2026Abstract We present updated hard X-ray polarization measurements of the Crab pulsar and nebula obtained with the balloon-borne polarimeter XL-Calibur in the ∼19–64 keV energy range. During the flight, intermittent failure of the Global Positioning System (GPS) receiver resulted in poorly constrained timing for ∼38% of the Crab dataset. By implementing a new phase recovery method that reconstructs timing during extended GPS-off intervals, phase tag data are recovered for ∼95% of the GPS-off dataset, increasing the precision of the phase-resolved analysis. Phase information for the data is recovered by using the Crab pulsar, with its 33 ms period, as an external timing source. Using a Markov Chain Monte Carlo framework to jointly fit phase offsets and frequency derivatives, sufficient phase accuracy is achieved across multiple periods without GPS for a phase-resolved analysis. This enables inclusion of nearly the full dataset in the polarization study. The polarization degree of the nebular emission is found to be (27.7 ± 4.9)% at a polarization angle of 127 2 ± 5 1, confirming previous XL-Calibur results and remaining aligned with the Crab’s spin axis, consistent with synchrotron emission from the inner nebula. Phase-resolved measurements show that the off-pulse and bridge intervals exhibit a strong polarization, while the pulsar peaks, although weakly constrained, remain in agreement with the softer-energy trends of IXPE. These findings reinforce a scenario in which hard X-ray emission arises primarily in the nebular torus and wind regions. The successful recovery of precise phase tagging from GPS-off data demonstrates the capacity to use the pulsar as an external clock even in the case of sparsely populated data.
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Publications of the Astronomical Society of Japan, Apr 9, 2026Abstract We report the presence of a highly ionized absorber in the transient, eclipsing low-mass X-ray binary AX J1745.6-2901, observed from 2024 February 26–29 with XRISM’s Resolve and Xtend instruments. During a soft/high state without dips, Resolve’s high spectral resolution ($E/\Delta E \sim 1000$, full width at half-maximum) revealed narrow velocity widths ($\sigma \sim 110~{\rm km~s^{-1 } }$) for Fe xxvi and Ni xxviii lines, even with low photon statistics. These widths are consistent with binary orbital motion. The observed modest blueshift velocity (${\sim }160~{\rm km~s^{-1 } }$) indicates that the absorber is located sufficiently far from the neutron star (${>}10^9$ cm), so that gravitational redshift effects are not dominant. On the other hand, broad-band spectral analysis using a photoionized plasma model applied to the Xtend data constrains the absorber to lie within a radius of ${\lesssim }10^{9.5}$ cm, as inferred from the upper limits of the best-fitting ionization parameter ($\log \xi \sim 4.4$) and the large column density (${\sim }1.6\times 10^{24}~{\rm cm^{-2 } }$). At this distance, the observed outward velocity of the absorber is about an order of magnitude smaller than the escape velocity from the neutron star.
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Journal of Astronomical Telescopes, Instruments, and Systems, 12(01), Mar 26, 2026
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Journal of Astronomical Telescopes, Instruments, and Systems, 12(01), Mar 14, 2026
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The Astrophysical Journal, 998(2) 210, Feb, 2026 Peer-reviewed
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Nature, 650(8101) 309-313, Feb, 2026 Peer-reviewed
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Publications of the Astronomical Society of Japan, 78(1) 97-110, Feb, 2026 Peer-reviewedAbstract A high-resolution X-ray spectroscopic observation was made of the RS CVn-type binary star HR 1099 using the Resolve instrument onboard XRISM for its calibration purposes. During the $\sim$400 ks telescope time covering 1.5 binary orbit, a flare lasting for $\sim$100 ks was observed with a released X-ray radiation energy of ${\sim }10^{34}$ erg, making it the first stellar flare ever observed with an X-ray microcalorimeter spectrometer. The flare peak count rate is 6.4 times higher than that in quiescence and is distinguished clearly in time thanks to the long telescope time. Many emission lines were detected in the 1.7–10 keV range both in the flare and quiescent phases. Using the high spectral resolution of Resolve in the Fe K band (6.5–7.0 keV), we resolved the inner-shell lines of Fe xix–xxiv as well as the outer-shell lines of Fe xxv–xxvi. These lines have peaks in the contribution functions at different temperatures over a wide range, allowing us to construct the differential emission measure (DEM) distribution over the electron temperature of 1–10 keV (roughly 10–100 MK) based only on Fe lines, thus without an assumption of the elemental abundance. The reconstructed DEM has a bimodal distribution, and only the hotter component increased during the flare. The elemental abundance was derived based on the DEM distribution thus constructed. A significant abundance increase was observed during the flare for Ca and Fe, which are some of the elements with the lowest first ionization potential among those analyzed, but not for Si, S, and Ar. This behavior is seen in some giant solar flares and the present result is a clear example in stellar flares.
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Nature Astronomy, 10 144-153, Jan, 2026 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77(6) 1278-1289, Dec, 2025 Peer-reviewed
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The Astrophysical Journall, 994(1) L28, Nov, 2025 Peer-reviewed
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The Astrophysical Journall, 993(1) L11, Nov, 2025 Peer-reviewed
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The Astrophysical Journal, 994(1) 37-37, Nov, 2025 Peer-reviewedAbstract The balloon-borne hard X-ray polarimetry mission XL-Calibur observed the black hole X-ray binary (BHXRB) Cygnus X-1 (Cyg X-1) during its nearly 6 day long-duration balloon flight from Sweden to Canada in 2024 July. The XL-Calibur observations allowed us to derive the most precise constraints to date of the polarization degree (PD) and polarization angle (PA) of the hard X-ray emission from a BHXRB. XL-Calibur observed Cyg X-1 in the hard state and measured a ∼19–64 keV PD of ( )% (equivalent to an upper limit, at the 99% level, of 11.1%) at a PA of −28° ± 17°, with an 8.7% chance probability of detecting larger PDs than the one observed, given an unpolarized signal. The XL-Calibur results are thus comparable to the 2–8 keV PD and PA found by Imaging X-ray Polarimetry Explorer (IXPE), with a similar agreement between the hard X-ray PA and the radio jet direction. We also discuss the implications of our polarization measurements in the context of models describing the origin of the broadband X-ray and γ -ray emission, to which XL-Calibur provides independent constraints on any proposed emission modeling.
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 044002, Oct, 2025 Peer-reviewed
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Nature, 646(8083) 57-61, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 042026, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 042023, Oct, 2025 Peer-reviewed
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Astronomy & Astrophysics, 702 A147, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 042003, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 042020, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 042008, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11(04) 042025, Oct, 2025 Peer-reviewed
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Journal of Astronomical Telescopes, Instruments, and Systems, 11(04) 042014, Oct, 2025 Peer-reviewedLead authorCorresponding author
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Journal of Astronomical Telescopes, Instruments, and Systems, 11 042016-56, Oct, 2025 Peer-reviewed
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UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXIV, 61-61, Sep 29, 2025
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UV, X-Ray, and Gamma-Ray Space Instrumentation for Astronomy XXIV, 51-51, Sep 18, 2025
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Optics for EUV, X-Ray, and Gamma-Ray Astronomy XII, 19-19, Sep 18, 2025
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Publications of the Astronomical Society of Japan, 77 S242-S253, Sep, 2025 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77 S193-S208, Sep, 2025 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77 S50-S62, Sep, 2025 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77 S23-S38, Sep, 2025 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77 S1-S9, Sep, 2025 Peer-reviewed
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The Astrophysical Journall, 988(2) L58, Aug, 2025 Peer-reviewed
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Journal of Instrumentation, 20(6) C06044, Jun, 2025 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77(Supplement_1) psaf063-S106, Jun, 2025 Peer-reviewedAbstract The Galactic Center region was observed with the X-Ray Imaging and Spectroscopy Mission (XRISM) X-ray observatory during the performance verification phase in 2024 and a point-like X-ray source was detected with the X-ray imager Xtend at a position of $(\alpha , \delta )_{\rm J2000.0}=({17^{\rm h}46^{\rm m}10{_{.}^{\rm s } }8}, {-29^{\circ}00^{\prime }21^{\prime \prime } })$, which is thus named XRISM J174610.8-290021. This source was bright in February to March and showed time variations in count rate by more than one order of magnitude in one week. The 2–10 keV X-ray luminosity was ${\sim }10^{35}$ erg s$^{-1}$ for the assumed distance of 8 kpc. However, after six months, it was below the detection limit. We found a hint of periodicity of 1537 s from timing analysis. The XRISM/Xtend spectrum has emission lines from helium-like iron (Fe xxv–He$\alpha$) at 6.7 keV and hydrogen-like iron (Fe xxvi–Ly$\alpha$) at 6.97 keV; their intensity ratio is unusual with the latter being four times stronger than the former. If the emission is of thermal origin, the ionization temperature estimated from the iron-line intensity ratio is $\sim$30 keV, which is inconsistent with the electron temperature estimated from the thermal bremsstrahlung, $\sim$7 keV. Spectral models of magnetic cataclysmic variables, which are often seen in the Galactic Center in this luminosity range, are found to fail to reproduce the obtained spectrum. By contrast, we found that the spectrum is well reproduced with the models of low-mass X-ray binaries containing a neutron star plus two narrow Gaussian lines. We consider that the source is intrinsically bright reaching $10^{37}$ erg s$^{-1}$, but is blocked from direct view due to a high inclination and only the scattered emission is visible. The photo-ionized plasma above the accretion disk with an ionization parameter of ${\sim }10^{5}$ may explain the unusual iron line ratio. We further discuss the potential contribution of point sources of the type of XRISM J174610.8-290021 to the diffuse Galactic Center X-ray emission.
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Monthly Notices of the Royal Astronomical Society, 540(1) L34-L40, Jun, 2025 Peer-reviewedABSTRACT We report measurements of the linear polarization degree (PD) and angle (PA) for hard X-ray emission from the Crab pulsar and wind nebula. Measurements were made with the XL-Calibur ($\sim$15–80 keV) balloon-borne Compton-scattering polarimeter in July 2024. The polarization parameters are determined using a Bayesian analysis of Stokes parameters obtained from X-ray scattering angles. Well-constrained ($\sim 8.5\sigma$) results are obtained for the polarization of the $\sim$19–64 keV signal integrated over all pulsar phases: PD = (25.1$\pm$2.9) per cent and PA = (129.8$\pm 3.2)^\circ$. In the off-pulse (nebula-dominated) phase range, the PD is constrained at $\sim 4.5\sigma$ and is compatible with the phase-integrated result. The PA of the nebular hard X-ray emission aligns with that measured by IXPE in the 2–8 keV band for the toroidal inner region of the pulsar wind nebula, where the hard X-rays predominantly originate. For the main pulsar peak, PD = (32.8$^{+18.2}_{-28.5}$) per cent and PA = (156.0 $\pm$ 21.7)$^\circ$, while for the second peak (inter-pulse), PD = (0.0$^{+33.6}_{-0.0}$) per cent and PA = (154.5 $\pm$ 34.5)$^\circ$. A low level of polarization in the pulsar peaks likely does not favour emission originating from the inner regions of the pulsar magnetosphere. Discriminating between Crab pulsar emission models will require deeper observations, e.g. with a satellite-borne hard X-ray polarimeter.
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Nature, 641(8065) 1132-1136, May, 2025 Peer-reviewed
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The Astrophysical Journall, 985(1) L20, May, 2025 Peer-reviewed
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The Astrophysical Journall, 982(1) L5, Mar, 2025 Peer-reviewed
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Nature, 638(8050) 365-369, Feb, 2025 Peer-reviewed
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Publications of the Astronomical Society of Japan, 77(1) L1-L8, Feb, 2025 Peer-reviewedAbstract Sagittarius A East is a supernova remnant with a unique surrounding environment, as it is located in the immediate vicinity of the supermassive black hole at the Galactic center, Sagittarius A$^{*}$. The X-ray emission of the remnant is suspected to show features of overionized plasma, which would require peculiar evolutionary paths. We report on the first observation of Sagittarius A East with the X-Ray Imaging and Spectroscopy Mission (XRISM). Equipped with a combination of a high-resolution microcalorimeter spectrometer and a large field-of-view CCD imager, we for the first time resolved the Fe xxv K-shell lines into fine structure lines and measured the forbidden-to-resonance intensity ratio to be $1.39 \pm 0.12$, which strongly suggests the presence of overionized plasma. We obtained a reliable constraint on the ionization temperature just before the transition into the overionization state, of $\gt\! 4\:$keV. The recombination timescale was constrained to be $\lt\! 8 \times 10^{11} \:$cm$^{-3}\:$s. The small velocity dispersion of $109 \pm 6\:$km$\:$s$^{-1}$ indicates a low Fe ion temperature $\lt\! 8\:$keV and a small expansion velocity $\lt\! 200\:$km$\:$s$^{-1}$. The high initial ionization temperature and small recombination timescale suggest that either rapid cooling of the plasma via adiabatic expansion from dense circumstellar material or intense photoionization by Sagittarius A$^{*}$ in the past may have triggered the overionization.
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The Astrophysical Journall, 977(2) L34-L34, Dec, 2024 Peer-reviewedAbstract The X-ray binary system Cygnus X-3 (4U 2030+40, V1521 Cyg) is luminous but enigmatic owing to the high intervening absorption. High-resolution X-ray spectroscopy uniquely probes the dynamics of the photoionized gas in the system. In this Letter, we report on an observation of Cyg X-3 with the XRISM/Resolve spectrometer, which provides unprecedented spectral resolution and sensitivity in the 2–10 keV band. We detect multiple kinematic and ionization components in absorption and emission whose superposition leads to complex line profiles, including strong P Cygni profiles on resonance lines. The prominent Fe xxv Heα and Fe xxvi Lyα emission complexes are clearly resolved into their characteristic fine-structure transitions. Self-consistent photoionization modeling allows us to disentangle the absorption and emission components and measure the Doppler velocity of these components as a function of binary orbital phase. We find a significantly higher velocity amplitude for the emission lines than for the absorption lines. The absorption lines generally appear blueshifted by ∼−500–600 km s−1. We show that the wind decomposes naturally into a relatively smooth and large-scale component, perhaps associated with the background wind itself, plus a turbulent, denser structure located close to the compact object in its orbit.
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Publications of the Astronomical Society of Japan, 76(6) 1186-1201, Dec, 2024 Peer-reviewedAbstract We present an initial analysis of the X-Ray Imaging and Spectroscopy Mission (XRISM) first-light observation of the supernova remnant (SNR) N 132D in the Large Magellanic Cloud. The Resolve microcalorimeter has obtained the first high-resolution spectrum in the 1.6–10 keV band, which contains K-shell emission lines of Si, S, Ar, Ca, and Fe. We find that the Si and S lines are relatively narrow, with a broadening represented by a Gaussian-like velocity dispersion of $\sigma _v \sim 450$ km s$^{-1}$. However, the Fe He$\alpha$ lines are substantially broadened with $\sigma _v \sim 1670$ km s$^{-1}$. This broadening can be explained by a combination of the thermal Doppler effect due to the high ion temperature and the kinematic Doppler effect due to the SNR expansion. Assuming that the Fe He$\alpha$ emission originates predominantly from the supernova ejecta, we estimate the reverse shock velocity at the time when the bulk of the Fe ejecta were shock heated to be $-1000 \lesssim V_{\rm rs}$ (km s$^{-1}$) $\lesssim 3300$ (in the observer frame). We also find that Fe Ly$\alpha$ emission is redshifted with a bulk velocity of $\sim 890$ km s$^{-1}$, substantially larger than the radial velocity of the local interstellar medium surrounding N 132D. These results demonstrate that high-resolution X-ray spectroscopy is capable of providing constraints on the evolutionary stage, geometry, and velocity distribution of SNRs.
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Nature Communications, 15(1) 7528, Sep, 2024 Peer-reviewed
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The Astrophysical Journall, 973(1) L25-L25, Sep, 2024 Peer-reviewedAbstract We present an analysis of the first two XRISM/Resolve spectra of the well-known Seyfert-1.5 active galactic nucleus (AGN) in NGC 4151, obtained in 2023 December. Our work focuses on the nature of the narrow Fe K α emission line at 6.4 keV, the strongest and most common X-ray line observed in AGN. The total line is found to consist of three components. Even the narrowest component of the line is resolved with evident Fe K α,1 (6.404 keV) and K α,2 (6.391 keV) contributions in a 2:1 flux ratio, fully consistent with neutral gas with negligible bulk velocity. Subject to the limitations of our models, the narrowest and intermediate-width components are consistent with emission from optically thin gas, suggesting that they arise in a disk atmosphere and/or wind. Modeling the three line components in terms of Keplerian broadening, they are readily associated with (1) the inner wall of the “torus,” (2) the innermost optical “broad-line region” (or “X-ray BLR”), and (3) a region with a radius of r ≃ 100 GM/c 2 that may signal a warp in the accretion disk. Viable alternative explanations of the broadest component include a fast-wind component and/or scattering; however, we find evidence of variability in the narrow Fe K α line complex on timescales consistent with small radii. The best-fit models are statistically superior to simple Voigt functions, but when fit with Voigt profiles the time-averaged lines are consistent with a projected velocity broadening of FWHM . Overall, the resolution and sensitivity of XRISM show that the narrow Fe K line in AGN is an effective probe of all key parts of the accretion flow, as it is currently understood. We discuss the implications of these findings for our understanding of AGN accretion, future studies with XRISM, and X-ray-based black hole mass measurements.
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray, 55-55, Aug 22, 2024
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray, 53-53, Aug 22, 2024
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray, 75-75, Aug 21, 2024
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray, 232-232, Aug 21, 2024
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Space Telescopes and Instrumentation 2024: Ultraviolet to Gamma Ray, 58-58, Aug 21, 2024
Misc.
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日本天文学会年会講演予稿集, 2025, 2025
Presentations
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Optics & Photonics Japan 2023, Nov 29, 2023 Invited
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44th COSPAR Scientific Assembly, 2022
Teaching Experience
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Sep, 2025 - Mar, 2026宇宙理学概論(オムニパス) (総合研究大学院大学)
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Sep, 2023 - Mar, 2024
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Apr, 2021 - Mar, 2022Space Science Review (The Graduate University for Advanced Studies)
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Oct, 2019 - Mar, 2020Space Science Review (The Graduate University for Advanced Studies)
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Oct, 2009 - Mar, 2010Applied Physics (Chuo University)
Professional Memberships
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Jul, 1998 - Present
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Mar, 1998 - Present
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Jan, 1997 - Present
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Aug, 1993 - Present
Research Projects
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2023 - Mar, 2027
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2022 - Mar, 2025
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Grants-in-Aid for Scientific Research, Japan Society for the Promotion of Science, Apr, 2022 - Mar, 2025
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Grants-in-Aid for Scientific Research Grant-in-Aid for Scientific Research (A), Japan Society for the Promotion of Science, Apr, 2020 - Mar, 2023
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科学研究費助成事業 挑戦的研究(萌芽), 日本学術振興会, Jun, 2019 - Mar, 2022
